1. Field of the Invention
This invention relates in general to a method and an apparatus for producing multi-color displays, and, more particularly, to a field sequential color shutter display system employing light polarizers and filters in conjunction with a liquid crystal cell combination.
2. Description of the Related Art
The present invention is an improvement over that invention disclosed and claimed in co-pending patent application Ser. No. 06/872,520 filed June 10, 1986, and now U.S. Pat. No. 4,770,500, of which I am a co-inventor and which is incorporated hereinto by reference.
The general idea of using a multi-color cathode ray tube in conjunction with liquid crystal cells and color polarizers has been described in numerous articles and issued patents such as the article by Brinson et al in IBM Technical Disclosure Bulletin Vol. 22, No. 5 of October 1979 and U.S. Pat. No. 4,582,396 to Bos et al.
In U.S. Pat. No. 4,582,396 to Bos et al., a typical field sequential color system is disclosed. A color sensitive polarizing means is placed in front of a cathode ray tube light source which is capable of emitting at least two colors. The polarizing means has a first absorption axis that passes linearly polarized light of the first color and a second absorption axis that passes linearly polarized light of the second color. A liquid crystal cell functions as a variable optical retarder in association with the polarizing filter means to selectively transmit a first or a second color, depending on the polarization of the light. The liquid crystal cell is followed by a linear polarizer.
When the liquid crystal cell is driven by a first signal it provides a half wave retardation to the applied light. When it is driven by a second signal, substantially no retardation is experienced by the impinging light. With substantially no retardation of the light, only light of one of the two colors can pass through the linear polarizer. With half wave retardation, only light of the other of the two colors can pass through the polarizer.
The device employs a specially designed liquid crystal cell to function as the variable retarder. A nematic liquid crystal cell is designed to be disclination-free and to switch in a "bounce-free" manner as it is switched between two states which alter the orientation of the surface non-contacting directors of the liquid crystal material in the cell.
The Bos et al. reference is typical of the prior art devices currently in use.
In these sequential color shutter systems, the image source, typically a cathode ray tube, emits light in a plurality of colors, for example, green and red. The bare cathode ray tube would then appear to emit yellow light (which is green and red combined) and would be viewed by an observer through a color shutter. The color shutter would permit information to be presented in either a red, green, or an intermediate color by changing light polarization states synchronously with the color information that is to be written. In its green state, the color shutter would transmit a certain percentage of green light, while absorbing a much higher percentage of red light, while the opposite would occur in the red state.
Intermediate colors, such as yellow, would be generated either by setting the shutter to an intermediate state where absorption in both primary colors would be more or less equal, or by "double-writing" yellow information in both of the primary colors.
In either case, systems of the type described above suffer from the relatively low transmission level of the selected color, which, in turn, leads to limited display brightness.
The present invention provides both a method and an apparatus to overcome the shortcomings of the prior art to produce a bright color display system suitable for use under both sunlight and night time conditions.
As a practical example of how the improved technology incorporated and found in both a device embodying the present invention, and the method of the present invention, may be used in current practical situations, the reader need only consider avionics multifunction display applications as used in tactical military aircraft.
In avionics systems such as this, which are capable of both raster and stroke display modes, the raster modes are often used for the display of monochrome sensor video information, while color information is presented in the stroke modes. Higher raster writing rates and the requirements for sunlight readability of multiple gray shade video-images make the (green) raster modes the more demanding of the two types of modes from a brightness/contrast viewpoint.
Reducing the red phosphor emission from the cathode ray tube (as is required for the improved configuration discussed below), proportionally increases the green light output from the cathode ray tube under the same drive conditions, thus raising the display efficiency in this color. While this configuration would reduce the red light output from the cathode ray tube, the overall red display luminance is enhanced by the high transmission of the color shutter in the red state as described in FIG. 4 below. Further, the brightness/contrast requirements for all colors of stroke symbology are often lower than those for green raster video presentations, thus resulting in an overall enhancement of system capability.